924 TjiAxsACTioxs OF THE Americax Institute. 



Great and Violent Storms. 



As to the causes, and ^tlie modes of generation of the great and 

 violent storms, I will suppose a case, which with modifications, may- 

 be taken as analogous to a majority of those which do occur. 



In the torrid zone, and during the hot seasons in the temperate 

 zones, a sultry heat with a tranquil and humid atmosphere, may be 

 taken as a precursor of storm. This may continue for days ; the 

 power of evaporation, being great, the air becomes more humid, 

 and, as Tyndall, Melloni, and others have demonstrated, humidity is 

 a powerful asborbent of heat ; it is, in a measure opaque to the heat- 

 ing rays ; it therefore, absorbs largely the sun's ray, as well as the 

 radiant and reflected heat from the earth. The more the air is. 

 heated the more its capacity for aqueous vapor is augmented, and 

 the greater is the absorption of heat. But although vapor absorbs 

 heat it radiates it also. Whence then can it radiate ? 



The lower stratum is superposed by strata which are saturated too ; 

 it may, therefore, radiate into vapor, but the vapor radiates into it, 

 also. The tendency then is for the air to retain its high degree of 

 heat ; to become rarified, and to rise ; and from some quarter cooler 

 air must come in to take its place. "What then must occur in our 

 ascending column of heated, humid air ? For a time the radiation is 

 intercepted, and, in great part, returned by the surrounding vapor; 

 condensation under such circumstances cannot take place. But the 

 quantity of aqueous vapor naturally diminishes as we ascend ; its 

 tension diminishes more rapidly than that of the air, and at length 

 the humid stratum finds itself above the protection wdiich had over- 

 spread it, and in the presence of purer space, where it pours its heat 

 into, and with but little return from, the interstellar ether. This free 

 radiation into space, together with the chilling efiect due to the 

 expansion of the ascending air, aflfords ample physical cause for the 

 condensation of vapor and the generation of clouds. Cumuli are now 

 formed, a stratum of which may extend over a large surface. Each 

 visible cumulus forms the capital of an invisible pillar of saturated 

 air. To take an extreme case, let us suppose that the sky being now 

 overcast and the process of cooling going on more rapidly^ another 

 stratum is formed lower down than the first, and then another still 

 lower. Cases are on record in which not less than three such strata 

 have been distinctly observed. Now, let it be remembered that each 

 stratum, in its relation to those above or below, maintains its own 

 independent insulation and electric individuality, the same as do the 



